Process for the production of a sheet of synthetic polymer fibrous material



D 1965 E. SOMMER ETAL 3,223,531

PROCESS FOR THE PRODUCTION OF A SHEET 0F SYNTHETIC POLYMER FIBROUSMATERIAL Filed Nov. 27, 1962 FIG! INVENTORS: ERWlN SOMMER KLAUS GERLACHHELMUT WERNER ATT'YS United States Patent 3,223,581 PROCESS FOR THEPRODUCTION OF A SHEET OF SYNTHETIC POLYMER FIBROUS MATERIAL ErwinSummer, Obernburg, Klaus Gerlach, Obernau, and

Helmut Werner, Elsenfeld, Germany, assignors to VereinigteGlanzstotf-Fabriken A.G., Wuppertal-Elberfeld, Germany Filed Nov. 27,1962, Ser. No. 240,342 Claims priority, application Germany, Nov. 30,1961, V 21,700 5 Claims. (Cl. 162-157) The present invention isconcerned with a process for producing sheet or paper-like structures ofsynthetic polymer fibers, and more particularly, the invention isconcerned with an improvement in the formation of fibrous sheets or webswhen laying or depositing fibers of a synthetic polymer from an aqueousdispersion onto a supporting surface and subsequently removing waterfrom the fibrous web.

Many dilferent fibrous materials have been employed for the productionof waterlaid sheet-like or paper-like structures according to thegenerally well-known papermaking process. Depending upon the exactnature of the initial fibrous material and the conditions under whichthe fibers are treated during the paper-making process, it is possibleto obtain sheet-like products with different properties andcharacteristics. Thus, paper is produced when using cellulosic fibers,and many attempts have been made to produce a similar sheet materialfrom fibers derived wholly from synthetic polymers.

In general, the over-all procedure for making paper or similar sheetmaterials is substantially the same in each case regardless of theparticular fiber, although there may be a number of minor variationssuch as pretreatment of the fibers by heating or refining or theincorporation of additives such as binders, adhesives, thickeningagents, fillers and the like.

Thus, in the usual paper-making process, the fibers are first dispersedin a liquid, preferably Water for obvious economical reasons, thedispersion of fibers being maintained by thorough mixing and/ or theaddition of a suitable dispersing agent. The resulting pulp or fiberdispersion is then transferred or flowed onto a moving screen or sievefor removal of at least part of the water and formation of a fibrous Webor fleece. The moving screen employed for this purpose corresponds toone of two gen eral types, the Fourdrinier machine and the cylindermachine. After the fibrous web has been formed, it is transferred fromthe moving screen onto other suitable apparatus for removal of anyremaining water and formation of a solid sheet by the application ofheat and/or pressure and/or adhesives. L4

The paper-making process is particularly adapted to the manufacture ofpaper from cellulosic fibers, and it has been quite difficult to treatfibers of synthetic polymers by this same process. A waterlaid fleece orfibrous web of synthetic polymer fibers cannot be transferred from theFourdrinier or similar moving screen without being torn or pulled apart,because there is very little cohesion between the individual polymerfibers. In contrast to natural cellulosic fibers which can be beateninto very fine fibers or fibrils, it has been practically impossible toprovide synthetic polymer fibers with similar properties capable offorming a relatively strong and cohesive waterlaid fibrous web. This ispartly due to the fact that it is very expensive to form fibrils fromsynthetic polymers, and partly due to the fact that synthetic polymerfibers have very low swelling values and a high degree ofwater-repellency.

In spite of numerous draw-backs, a number of attempts have been made tocarry out processes for making sheet 3,223,581 Patented Dec. 14, 1965materials containing synthetic polymer fibers in order to obtain theadvantages of various physical and chemical properties from suchpolymers as polyamides, polyesters, polyolefines and the like. Accordingto one known process, a sheet of synthetic polymer fibers is made byadding to the aqueous dispersion a relatively large proportion ofcellulose fibers, for example, in amounts up to about 30% by weight. Inthe same manner, cellulosic fibers have been modified with minor amountsof synthetic polymer fibers in order to impart certain properties to theresulting paper product. It will be obvious that such procedures do nottake full advantage of the preferred properties of the synthetic polymerfibers, and the admixture of very small amounts of a cellulose pulp doesnot overcome the disadvantages in working with the waterlaid fibrousweb.

Another process suggested by the prior art consists in the addition ofcertain adhesives or viscosity-increasing agents to the fiber dispersionin order to increase the strength of the fibrous web. A typical exampleof this procedure is the addition of carboxymethyl cellulose in anamount of about 0.1-5 by weight to the aqueous dispersion of the fibers.It will be evident that the fiber dispersion then contains about partsby weight or more of the additive for each part by weight of fibers.Since the additive, such as carboxymethyl cellulose, is generally atleast partly soluble in water, large amounts of the additive will 'beremoved as the fibrous web is drained or dewatered, and it is thereforeessential to recirculate the water in order to avoid substantial loss ofthe additive. Furthermore, the carboxymethyl cellulose or other additiveremaining in the fibrous web must be washed out during subsequentprocessing steps or at least removed from the finished product, sincesuch additives impair the quality, color, or other desirable propertiesof the final product.

Still another technique which has been suggested for the production ofsheet structures of synthetic polymer fibers is the addition to thefiber dispersion of so-called fibrids. These fibrids may becharacterized as ultra-fine fibers which can be obtained, for example,when a solution of a synthetic polymer is sprayed with high velocityonto the surface of a precipitation bath. The production of thesefibrids and their use in the formation of waterlaid fibrous Webs fromaqueous dispersions is described in detail in US. Patent No. 3,062,702.The use of such fibrids in the paper-making process does result in astronger and more easily handled fibrous web, and it is possible in thismanner to produce a sheet composed solely of synthetic polymer fibers.However, the methods required for the production of the fibrids arequite complicated and very expensive, particularly because large amountsof solvents and precipitating agents must be circulated and treatedduring production and recovery of the fibrids.

Finally, processes have been developed for the socalled fibrillation ofsynthetic polymer fibers. In this case, an attempt is made to duplicatethe effect of the beater in the usual paper-making process whereincellulose fibers are beaten in order to fibrillate and hydrate the pulpmaterial. However, synthetic polymer fibers cannot be hydrated, and anyfibrillation is achieved only by using very expensive beating methods.Furthermore, as in the production of fibrids, it may be necessary todeviate substantially from the usual filament spinning processes inorder to achieve satisfactory fibrillation.

Although it has been possible with prior processes to produce fibrousfleeces or waterlaid webs which can be transferred from the paper-makingmachine without being torn or pulled apart, the various disadvantages ofthese processes prevent a practical or economical production of asynthetic polymer fibrous sheet material.

A primary object of the present invention is to provide a new andimproved process for the production of a sheet of synthetic polymerfibrous material wherein the waterlaid fleece or web of fibers issufliciently strong so that it can be transferred from the screen of thepaper-making machine and otherwise handled during subsequent treatmentwithout being ruptured or pulled apart.

Another object of the invention is to provide a commercially practicaland economical process for the felting or waterlaying of syntheticpolymer fibers in which it is possible to obtain the advantages ofconventional filament spinning methods and paper-making processes.

Still another object of the invention is to provide a waterlaid orfelted product consisting substantially or wholly of synthetic polymerfibers in the form of a strong, continuous water-containing web orfleece, and it is also an object of the invention to produce thisproduct without employing special adhesives or thickening agents.

These and other objects and advantages of the invention will become moreapparent from the following detailed description and the accompanyingdrawing where- FIGS. 1 and 2 represent a cross-section ofintercontacting synthetic polymer fibers as employed for the purposes ofthis invention; and

FIG. 3 is a schematic representation of suitable papermaking apparatusfor carrying out the process of this invention.

It has now been found, in accordance with the invention, that a sheet orpaper-like structure can be readily produced from fibers of syntheticpolymers, by forming a fibrous web on a supporting surface, such as theusual screen or sieve of a paper-making machine, the fibrous web beingformed with individual fibers which consist predominantly of syntheticpolymer strands having smooth and flat surfaces, whereby the individualfibers are in contact with each other along these flat surfaces. Afterinitial formation of the fibrous web and before its trans fer from thesupporting screen, it is essential to withdraw water therefrom to aresidual moisture content of about A 30 to 85% by weight with referenceto the wet web.

In general, the process steps correspond to the usual paper-makingprocedure in which the fibers are first trans ferred from an aqueousdispersion to a screen or similar supporting surface of a paper-makingmachine, water is then withdrawn from the waterlaid and interlacedfibers to form a fibrous web or fleece on the supporting surface, andthe web is then removed from the supporting surface and finally formedinto a solid sheet of bonded fibrous material by heating, applyingpressure and/or applying various well-known sprays, plastic or resindispersions, adhesives or the like. The web containing a 30-85% residualmoisture content can be more fully dried with removal of part or all ofthe remaining water just prior to solidification of the web, or theresidual moisture can i also be removed during or after solidificationor bonding of the fibers in the web. However, a residual moisturecontent of 30-85% is essential in combination with the particularsynthetic polymer strands of the invention in order to produce awaterlaid web or sheet on the papermaking machine with suflicientstrength or cohesiveness for subsequent removal of the web from thesupporting screen without tearing or otherwise substantially damagingthe web.

The exact form or shape of the individual fibrous strands is a criticalfeature of the present invention, since it is essential that thesynthetic polymer strands have a large area of surface contact with eachother in the waterlaid fibrous web. FIGS. 1 and 2 of the drawingillustrate two types of such synthetic polymer strands which meet thenecessary conditions of the invention with respect to surface area andshape. FIG. 1 illustrates fibers in the form of bands or tapes, having asubstantially rectangular cross-section, and it is apparent how the fiatsurfaces of these small bands contact each other in the waterlaid sheet.The same result is achieved with fibers as represented in cross-sectionin FIG. 2, wherein the strands have an approximately triangularcross-section. It will be self-evident that fibrous synthetic polymerstrands of other types are also suitable provided that they presentessentially smooth and flat or planar surfaces, thereby providing alarge area of contact between fiber surfaces. Of course, the mostfavorable conditions will exist when the ratio of the cross-sectionalcircumference to the cross-sectional area is as great as possible.

In general, all fiber-forming or film-forming synthetic polymers can beused in order to form the flat-surfaced strands required for thepurposes of this invention. For example, such polymers includepolyamides such as nylon and polycaprolactam, polyesters such aspolyethylene terephthalate, polyolefines such as polyethylene andpolypropylene, and polymers or copolymers of acrylonitrile, vinylchloride, vinylidine chloride and the like. It will be recognized thatthe structure or shape of the synthetic polymer fibrous strand is theimportant consideration, and the chemical composition of the polymer isimportant only because synthetic polymer fibers as a class cannot behydrated or subjected to fibrillation according to the usualpaper-making procedure with natural or artificial cellulose fibers.

Synthetic polymer fibrous strands having the required shape or form canbe obtained quite easily by spinning the molten polymer through a nozzleorifice or opening having the desired cross-sectional shape. Forexample, strands, having the shape shown in FIG. l can be obtained byextrusion through a nozzle with a substantially rectangular slit-shapedopening. A triangular nozzle opening will produce the cross-section ofthe fibrous strand in FIG. 2, e.g., an isosceles triangle. Furthermore,it is possible to obtain such fibrous strands by cutting up thin foilsor films of the synthetic polymer. As indicated by the drawing, thecross-sectional shape of the polymer strand need not be absolutelyuniform or correspond to an exact geometric configuration, and slightvariations are permitted within the scope of the invention, e.g.,rounded edges or somewhat irregular surfaces. Thus, as other examples ofsuitable structures, the strands may have the form of flattened ovals orequilateral triangles or narrow trapezoids. The most favorable resultsare obtained when the polymer strand has two oppositely disposed flatfaces or planar surfaces and relatively smaller or narrower edgesseparating the two faces. Also, the length of the cross-section shouldbe relatively large by comparison to the width, and it is preferred tohave a ratio of maximum length to maximum width of the cross-section ofat least 3:1 or more. In general, this ratio of maximum length tomaximum width should fall within a range of 3:1 to 10:1.

The denier of the fibers or strands according to the invention should beas low as possible and should amount to not more than about 3 denier,preferably about 0.8 to 2.0 denier. The length of the individual strandsis dependent upon the properties or characteristics desired in thefinished sheet product. In general, however, the synthetic polymerstrands should have a length of about 1 to 10 mm., preferably 2 to 6 mm.

The web or fleece which is waterlaid from an aqueous dispersion shouldconsist predominantly of fibrous strands having the above-describedshapes or configurations, and it will be obvious that one may usemixtures of difierently shaped fibers as well as mixtures of fibersprepared from different synthetic polymers. The present invention isespecially useful for producing waterlaid webs or fleeces which consistwholly of synthetic polymer fibers. In any case, the invention isdirected especially to waterlaid webs containing at least 90% by weightand preferably 95% by weight of synthetic polymer fibers with respect tothe total weight of fibers in the web.

Although consisting wholly or substantially wholly of synthetic polymerfibers, the waterlaid web and the finished sheet product may alsocontainsmall amounts of other fibers or such materials as fillers, binders,pigments or the like. However, such additives should ordinarily be.present in amounts of not more than and preferably not more than 5% byweight with reference to the dry web. With the process of the invention,it is unnecessary to add any adhesive or bonding agents .until after thewaterlaid web has been formed and transferred from the screen or sieveof the paper-making machine onto .other apparatus for solidificationinto a A permanent sheet material.

If the spinning process for the production of the synfiber pulp which isto be transferred to the paper-making The actual manner in which thesynthetic polymer strands are formed into a sheet structure according tothe invention can be described as follows. About 0.01 to 0.5 andpreferably 0.05 to 0.1 parts by weight of the polymer strands arefirstdispersed in 100 parts by weight of water, in a conventional mixingbox or pulp tank 1. The fibrous dispersion is preferably achieved by theaddition of a small amount of any conventional dispersing agent togetherwith intense mixing or rapid agitation of the pulp. The alkane sulfonatetype of dispersing agent can be readily employed, these agents beingcharacterized by a sulfonate group attached to a long chain aliphaticradical as the hydrophobic group.

Such compounds are typical anionic surface active agents as .described,for example, in chapter 4 of Surface Active Agents, by Schwartz andPerry, Interscience Publishers, Inc. (1949). The alkane sulfonateshaving an ester intermediate linkage are especially useful, for examplein which a sulfonated lower alkanol is esterified with a long chainalphatic carboxylic acid such as oleic,

stearic and palmitic acids or other fatty acids and their closelyrelated derivatives. In general, these dispersing agents are obtainedcommercially in the form of their sodium salts of the formula RCOOC H SONa in which R is the residue of a fatty acid such as oleic acid. Thesedispersing agents are sold under the trade name Igepon A, AP andAPExtra. Other suitable dispersing agents for the purposes of thisinvention are the nonionic compounds which .are obtained by the actionof ethylene oxide on highmolecular alcohols (e.g., oleyl alcohol) andwhich have the. formula R(OC H ,OH

where R is a long chain aliphatic residue and n is a number between 9and 20. Such compounds are wellknown under the trade names Emulphor O,Emullat, Cirrasol SF, and Peregal G, for example.

The aqueous dispersion or fiber pulp is discharged or poured from themixing box 1 through a slit-shaped or elongated opening 2 onto theso-called wire or endless screen belt 3 of a conventional paper-makingmachine such as the Fourdrinier. A Fourdrinier machine is preferred withits vibrating or shaking action which tends to orient the syntheticpolymer strands so as to align and intercontact the flat surfacesthereof. As the layer of fiber pulp is conveyed as a fleece or web F bythe wire or endless screen or any similar perforated supporting surface,water is drained off by means of suitable suction boxes 4 and squeezerollers 5. If desired, other rollers may also be provided to work thefibrous pulp into a relatively uniform waterlaid fleece or web.

Sufiicient water is removed from the fleece or web F so that by the timeit leaves the endless screen 3, for example at point 6, it has aresidual moisture content of 30 to 85% by weight, and preferably 60 to75% by weight, with reference to the weight of the wet fleece. With thisresidual moisture content, the fleece is then transferred to a secondconveyor belt in the form of a second enddoes not tear apart. .besolidified by bonding the individual polymer strands does it tear. ofapparatus not represented in the drawing and conto each other byconventional means not shown in the drawing. This solidification can beeasily accomplished by the application of pressure and/or heat with orwithout the application of various adhesive or bonding agents which arewell-known in the art. Fillers, pigments or the like may also be addedat the same time that the waterlaid fibrous web is solidified. The termsolidification is employed herein with reference to the permanentbondingof the individual strands to each other as distinct from thesolidification of a molten material or the adherence of the water-coatedflat surfaces of the individual fibrous strands in the waterlaid web.

The process of the invention is further illustrated with additionaldetail by the following examples. Parts and percentages in-theseexamples are by weight unless otherwise indicated. It will be understoodthat the examples are illustrative only and that the invention shouldnot be limited thereby.

Example 1 Polycaprolactam is melted in the usual manner and is spunthrough a 36-hole nozzle with a rectangular crosssection of each hole oropening of x 600 my. The drawing-off speed of the extruded strands is1000 m./min. After stretching in a ratio .of 1:3, the resulting strandshave an individual fiber denier of 2 and a cross-section as shown inFIG. 1. The strands are then freed from excess spinning preparation bywashing with a soap solution and are cut to a length of 5 mm.

A 0.05% aqueous fiber dispersion is prepared by mixing the strands inwater together with several cubic centimeters of a 10% aqueous solutionof a sodium salt of a higher aliphatic sulfonic acid of the formula as adispersing agent. The dispersion is homogenized by intense agitation andis then processed with the apparatus shown schematically in FIG. 3. Thefiber dispersion is first discharged from mixing box 1, throughslit-shaped opening 2 ontothe moving endless screen 3, on which thefiber pulp is worked into a fleece or web F. The screen is equippedwith'suction boxes 4 and squeeze rollers 5. By means of this apparatus,water is removed from the fleece so that is 6, i.e., upon leaving thescreen 3, the fleece has a residual moisture content of 65% (withreference to the weight of the wet fleece). The fleece is nowtransferred to a second conveyor screen 7, and during this transfer, thefleece does not lengthen nor Further processing takes place by meanssists of spraying the fleece with a 10% aqueous dispersion of polyvinylchloride, and drying the fleece .or web without application of pressureat about 'C. A soft liner material is obtained as the final resin-bondedfibrous sheet.

Example 2 Corresponding to the data of Example 1, polyethyleneterephthalate is spun from the melt with the use of a 36-hole nozzle.The rectangular cross-section of the nozzle openings is 60 x 800 my. Thedrawing-01f speed amounts to 500 m./min. Stretching is carried out in aratio of 1:4, and the resulting strands are obtained with an individualfiber denier of 1.2, the cross-section again appearing as in FIG. 1.After washing with a soap solution to remove adherent spinningpreparations or finishing agents, the strands are cut to a length of 6mm.

With addition of the dispersing agent described in Example 1, a 0.05aqueous fiber dispersion is prepared and is further processed on thepaper-making machine according to the data of Example 1. The fiberfleece leaving the Fourdrinier screen has a residual moisture content of70% (with reference to the weight of the wet fleece). After beingtransferred to the second conveyor screen, the fleece is sprayed with aaqueous dispersion of polymethacrylic acid ester. The fleece or web isfinally dried without application of pressure at a temperature of about110 C. so as to provide a soft liner material consisting of a bondedfibrous sheet.

Example 3 A polyamide of adipic acid and hexamethylene diamine (nylon)is spun from the melt and stretched under the same conditions describedin Example 2. The strands in this case likewise have an individual fiberdenier of 1.2 and are cut to a length of 6 mm.

A 0.05% aqueous dispersion of these strands is prepared with the samedispersing agent mentioned in Example 1. After leaving the screen 3 ofthe paper-making machine, the residual moisture content of the waterlaidfleece is 75%. The solidification of the fleece is accomplished byspraying With a 5% methanolic copolyamide solution (the copolyamideconsisting of 60% hexamethylene diamine adipate and caprolactarn).Drying is then carried out at about 70 C., and the web is subsequentlypressed at about 120 C. and a pressure of 5 kg./cm. The final sheetproduct has a smooth paper-like structure.

Example 4 A melt of polypropylene is spun through a 36-hole nozzle witha rectangular cross-section of the nozzle openings of x 800 my. at adrawing-off speed of 500 m./rnin. Stretching is carried out in a ratioof 1:3, and the resulting strands have an individual fiber denier of 1.3and are cut to a length of 5 mm.

From these strands, there is prepared a 0.08% aqueous dispersion, makinguse of the dispersing agent described in Example 1. The fibrousdispersion is then processed on the paper-making apparatus as in Example1 to a residual moisture content of The fleece is finally sprayed with a10% aqueous dispersion of :polyrnethacrylic acid ester, dried at aboutC. and thereupon pressed at C. and under a pressure of 5 kg./cm. Asmooth, soft, paper-like web is obtained in which the individual strandsare bonded together.

From the foregoing examples, it will be apparent that the presentinvention permits a very economical production of sheet or paper-likestructures of synthetic polymer fibrous materials while usingconventional papermaking apparatus. The intermediate waterlaid fibrousfleece or web is characterized by its unusual strength and resistance todeformation so that subsequent treatment steps such as drying andsolidification by bonding can be carried out without any difficulties.Because of this ease in handling the waterlaid web, the final orfinished product in the form of a solidified sheet or paper-likestructure has a much more uniform quality and strength than could beachieved in prior processes. The process of the invention thus providesa greater opportunity for making useful articles which consist wholly orat least predominantly of waterlaid and bonded synthetic polymer fibers.

The invention is hereby claimed as follows:

1. In a process for the production of a sheet of synthetic polymerfibrous material wherein an aqueous dispersion of synthetic polymerfibers is transferred to a supporting surface, water is withdrawn toform a fibrous web on said supporting surface, and the web is thenremoved from said supporting surface and formed into a bonded sheet offibrous material, the improvement which comprises: forming a fibrous webon said supporting surface with individual fibers which consistpredominantly of melt-spun, non-fibrillated synthetic polymer strands,each of said strands having a solid elongated cross-section in which theratio of maximum length to maximum width is at least 3:1 and each ofsaid strands also having smooth and fiat surfaces, whereby saidindividual fibers are in contact with each other along said flatsurfaces; withdrawing water from said fibrous web to a residual moisturecontent of 30 to 85% by weight with reference to the wet web; andremoving said fibrous web with said moisture content from the supportingsurface.

2. A process as claimed in claim 1 wherein the length of the individualstrands is about 2 to 6 mm.

3. A process as claimed in claim 1 wherein the individual syntheticpolymer strands have a denier of not more than about 3.

4. A process as claimed in claim 1 wherein the individual syntheticpolymer strands have the shape of a band with an elongated,substantially rectangular crosssection.

5. A process as claimed in claim 1 wherein the individual syntheticpolymer strands have the shape of a band with an elongated substantiallytriangular crosssection.

References Cited by the Examiner UNITED STATES PATENTS 2,810,646 10/1957Wooding et a1. 162-146 2,999,788 9/1961 Morgan 162157 3,002,880 10/1961Schonberg et al. 162157 3,068,527 12/1962 Morgan 162-146 3,081,5193/1963 Blades et a1. 162-157 OTHER REFERENCES Calkin, Modern Pulp andPaper Making, 3rd edition, 1957, Reinhold Publication Corp., New York,page 312.

DONALL H. SYLVESTER, Primary Examiner.

' MORRIS o. WOLK, Examiner.

1. IN A PROCESS FOR THE PRODUCTION OF A SHEET OF SYNTHETIC POLYMERFIBROUS MATERIAL WHEREIN AN AQUEOUS DISPERSION OF SYNTHETIC POLYMERFIBERS IS TRANSFERRED TO A SUPPORTING SURFACE, WATER IS WITHDRAWN TOFORM A FIBROUS WEB ON SAID SUPPORTING SURFACE, AND THE WEB IS THENREMOVED FROM SAID SUPPORTING SURFACE AND FORMED INTO A BONDED SHEET OFFIBROUS MATERIAL, THE IMPROVEMENT WHICH COMPRISES: FORMING A FIBROUS WEBON SAID SUPPORTING SURFACE WITH INDIVIDUAL FIBERS WHICH CONSISTPREDOMINANTLY OF MELT-SPUN, NON-FIBRILLATED SYNTHETIC POLYMER STRANDS,EACH OF SAID STRANDS HAVING A SOLID ELONGATED CROSS-SECTION IN WHICH THERATIO OF MIXIMUM LENGTH